Examples of a known seat reclining apparatus are disclosed in JP2002-282078A and in JP2002-101996A. The seat reclining apparatus disclosed in JP2002-282078A includes a lower arm, an upper arm and a lock mechanism. The lower arm is retained by a seat cushion and includes two pawls, each of which has external teeth at an end portion thereof. The upper arm is rotatably supported by the lower arm and is fixed at a seat back. Furthermore, internal teeth are formed at the upper arm so as to be engageable with and disengageable from the external teeth of each of the pawls. The lock mechanism includes a cam and a spring for locking the engagement between the upper arm and each of the pawls.
In order to adjust an inclining angle of the seat back, a user needs to pull up an operation lever in order to rotate the cam in an unlocking direction so as to resist against a spring force of the spring to displace the pawls in a radially inward direction of the lower arm and disengage the external teeth of the pawls from the internal teeth of the upper arm. As a result, the engagement between the upper arm and each of the pawls is unlocked. Then, after the inclining angle of the seat back is adjusted, the cam rotates in a locking direction by the spring force of the spring in order to displace the pawls in a radially outward direction of the lower arm, thereby engaging the external teeth of the pawls with the internal teeth of the upper arm. As a result, the engagement between the upper arm and each of the pawls is locked. A pair of supporting portions, each of which is formed in an arc shape when being viewed from front and each of which protrudes in a direction corresponding to a rotational axis, is formed at an intermediate portion of the lower arm so as to face each other in a radial direction of the lower arm. The cam includes a pair of elongated grooves, each of which is formed in an arc and into which the corresponding supporting portions are freely engaged. Accordingly, the cam is allowed to rotate while avoiding an eccentricity thereof by the pair of the supporting portions.
The seat reclining apparatus disclosed in JP2002-101996A includes a lower arm, an upper arm and a lock mechanism, which are similar to the lower arm, the upper arm and the lock mechanism of the seat reclining apparatus disclosed in JP2002-282078A. The seat reclining apparatus disclosed in JP2002-101996A differs from the seat reclining apparatus disclosed in JP2002-282078A in that the seat reclining apparatus disclosed in JP2002-101996A includes three pawls. Three guide portions, each of which is formed in a trapezoidal shape when being viewed from front and each of which protrudes in a direction corresponding to a rotational axis, are formed at the lower arm while keeping an equal angular distance (approximately 120 degrees) between the neighboring guide portions about the rotational axis. The guide portions guide a movement of the pawls in a radial direction at wall surfaces, which extend in the radial direction, of the corresponding guide portions. Furthermore, the guide portions normally guide an outer circumferential surface of the cam at arc-shaped inner wall surfaces of the corresponding guide portions. Accordingly, the cam is allowed to rotate while avoiding an eccentricity thereof.
For example, a seat reclining apparatus, which has a high engagement strength of the pawls relative the lower arm and whose size and weight is relatively small, is required for a vehicle seat. In order to increase the engagement strength of the pawls relative to the lower arm, for example, a number of pawls may need to be increased (i.e. additional pawls may be added to the seat reclining apparatus). However, according to the seat reclining apparatus disclosed in JP2002-282078A, the supporting portions for restricting the eccentricity of the cam need to be provided at the lower arm and the grooves into which the corresponding supporting portions are inserted need to be formed at the cam. Therefore, a size of the cam in the radial direction thereof needs to be enlarged in order to ensure a rigidity of the cam, which may result in an increase of the seat reclining apparatus, specifically, a size of the seat reclining apparatus may be increased in a rotational radial direction. Furthermore, the arc-shaped elongated grooves, into which the corresponding supporting portions are freely engaged, need to be formed at the cam. Therefore, in the case where the number of the pawls is increased, the number of elongated grooves also needs to be increased, which my result in decreasing the rigidity of the cam. As a result, the engagement of the pawls relative to the lower arm may not be maintained. On the other hand, according to the seat reclining apparatus disclosed in JP2002-101996A, the eccentricity of the cam is avoided in a manner where the outer circumferential surface of the cam slides against a surface of protrusions, which face the axis of the seat reclining apparatus and which are formed at the lower arm. Therefore, the seat reclining apparatus disclosed in JP2002-101996A does not need to include the supporting portions and the groove portions of the seat reclining apparatus disclosed in JP2002-282078A at the lower arm and the cam, respectively. Accordingly, the seat reclining apparatus disclosed in JP2002-101996A may not cause a reduction in the rigidity of the cam or an increase of a size of the seat reclining apparatus in the radial direction.
However, according to the seat reclining apparatus disclosed in JP2002-101996A, in a case where, for example, a number of the pawls is increased to four from three within a limited space, four guide portions, each of which is formed in a triangle shape when being viewed from front, need to be formed at the lower arm so as to protrude in the direction extending along the rotational axis while keeping an equal angular distance (approximately 90 degrees) between the neighboring guide portions about the rotational axis because of the limited space. The guide portions may guide the movement of the pawls in the radial direction by the wall surfaces of the corresponding portions extending in the radial direction. However, an inner circumferential wall surface of each of the guide portions for guiding the rotation of the cam is positioned at a corner of the triangle shape thereof. Therefore, an area of the arc-shaped wall surface of each of the guide portions for guiding the outer circumferential surface of the cam may be reduced. As a result, the guide portions may not normally restrict the eccentricity of the cam when rotating. In order to normally restrict the eccentricity of the cam when rotating, an additional and separate eccentricity restricting member may need to be provided at the seat reclining apparatus. However, providing the additional and separate eccentricity restricting member may lead to an increase of a size and a weight of the seat reclining apparatus, which may further result in an increase of manufacturing costs of the seat reclining apparatus.
A need thus exists to provide a seat reclining apparatus which is not susceptible to the drawback mentioned above.